An ordinary plant control device is configured to determine the control input of the plant by feedback control so as to cause the output value of a control amount to follow a target value when the target value is given in regard to the control amount of the plant. However, in actual control of the plant, various constraints are often present in hardware or control in regard to the state quantity of the plant. If these constraints are not satisfied, there is a concern of occurrence of breakage of the hardware and reduction in the control performance. Satisfiability of constraints is one of important performances which are requested in control of the plant, as well as followability of the output value to the target value.
A reference governor is effective means for satisfying the above described request. A reference governor includes a prediction model which is obtained by modeling a closed loop system (a feedback control system) including a plant which is a control target and a feedback controller, and predicts a future value of the state quantity on which a constraint is imposed according to the prediction model. Subsequently, the reference governor corrects the target value of the control amount of the plant based on the predicted value of the state quantity and the constraint imposed on the state quantity.
What is considered as important in design of the reference governor is the content of the evaluation function for use in calculation of a corrected target value. If consideration is given to only satisfiability of the constraint, various evaluation functions can be adopted. However, depending on what evaluation function is used to determine the corrected target value, the following characteristic of the output value to the target value of the control amount significantly changes, and the transient response characteristics of the output value to the change of the target value of the control amount also significantly changes. Especially in the case of the plant in which the response characteristics vary in response to the operation conditions as in the internal combustion engine for an automobile, the transient response characteristics of the output value to the change of the target value of the control amount significantly vary depending on the operation conditions. To enable favorable transient response characteristics to be achieved while satisfying a constraint is an important problem for a plant control device, in particular, a plant control device the control target of which is a plant in which the response characteristics change in response to the operation conditions.
As an example of the prior art which applies a reference governor to the control of a plant, the prior art disclosed in Patent Literature 1 as follows can be cited. The prior art relates to tension control of the rolled steel material in a multistage rolling mill. In the prior art disclosed in Patent Literature 1, the target orbit data specifying the temporal change of the tension of the rolled steel material is calculated in advance by the reference governor, and the tension of the rolled steel material is controlled based on the deviation between the actual tension value of the rolled steel material and the target orbit data. However, in this prior art, the control target is not a plant in which the response characteristics change in response to the operation conditions, and calculation of the target orbit data by the reference governor is performed offline. Therefore, even if the prior art is applied to the plant in which the response characteristics change in response to the operation conditions, the prior art cannot solve the problem of achieving favorable transient response characteristics while satisfying a constraint.